Extreme, passionate belief in something that makes people act unreasonably or even dangerously.
From 'fanatic' (from Latin 'fanaticus' meaning 'inspired by a deity,' from 'fanum' meaning 'temple'). Ironically, the word started meaning 'divinely inspired' but now means excessive zeal regardless of reason.
The difference between passion and fanaticism is nuanced—a passionate fan loves their team; a fanatic might do something harmful for it—and brain scans show that fanaticism actually activates different neural networks than normal enthusiasm, making it physiologically distinct!
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